Resonance, and stabilization

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

These acids (51) are organic molecules that contain a plurality of cyano groups and are readily ionized to hydrogen ions and resonance-stabilized anions. Typical cyanocarbon acids are cyanoform, methanetricarbonitrile (5) 1,1,3,3-tetracyanopropene [32019-26-4] l-propene-l,l,3,3-tetracarbonitrile (52) 1,1,2,3,3-pentacyanopropene [45078-17-9], l-propene-l,l,2,3,3-pentacarbonitrile (51) l,l,2,6,7,7-hexacyano-l,3,5-heptatriene [69239-39-0] (53) 2-dicyanomethylene-l,l,3,3-tetracyanopropane [32019-27-5] (51) and l,3-cyclopentadiene-l,2,3,4,5-pentacarbonitrile [69239-40-3] (54,55). Many of these acids rival mineral acids in strength (56) and are usually isolable only as salts with metal or ammonium ions. The remarkable strength of these acids results from resonance stabilization in the anions that is not possible in the protonated forms. 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

Scheme 9.3. Correlation between for Retro-Diels-Alder Reaction and Resonance Stabilization of Aromatic Products...

The concepts of destabilization of reactants and stabilization of products described for pyrophosphate also apply for ATP and other phosphoric anhydrides (Figure 3.11). ATP and ADP are destabilized relative to the hydrolysis products by electrostatic repulsion, competing resonance, and entropy. AMP, on the other hand, is a phosphate ester (not an anhydride) possessing only a single phosphoryl group and is not markedly different from the product inorganic phosphate in terms of electrostatic repulsion and resonance stabilization. Thus, the AG° for hydrolysis of AMP is much smaller than the corresponding values for ATP and ADP. 

In all the examples studied, the difference in the free energy between the anhydrous and hydrated species is 4 kcal/mole or less. ° Both electron deficiency and resonance stabilization are necessary for covalent hydration to be measurable. The necessity for electron deficiency is clearly shown in the following examples. The cation of 1,4,5-triazanaphthalene is anhydrous, but the cation of 1,4,5,8-tetraazanaphthalene is predominantly hydrated. 1,6-Naphthyridine cation is anhydrous, whereas the cations of the 3- and 8-nitro derivatives are predominantly hydrated. Also, the percentages of the hydrated form in the neutral species of 2-hydroxy-1,3-diaza-, 1,3,8-... 

The first generalization is illustrated by the behavior of the 2- and 4-vs. the 3-derivatives of pyridine, the second by the reactivity of 4- vs. 2-substituted pyridines, the third by the relation of 4- vs. 2-derivatives of pyrimidine, and the fourth by the appreciable reactivity of 3-substituted pyridines or 5-substituted pyrimidines compared to that of their benzene analogs. Various combinations of azine-nitrogens in other poly-azines supply further examples. Theoretical aspects of (1), (2) and (3) are discussed in Section II, B, 2. The effect involved in (4) is believed to be more the result of the inductive stabilization of an adjacent negative chaise in the transition state (cf. 251) than of the electron deficiency created in the ground state (cf. 252). The quantitative relation between inductive stabihzation and resonance stabilization is not precisely defined by available data. However, a... 

Flat-oriented bipyridinium compounds adsorb with additional stabilization compared to flat-oriented mono-pyridinium compounds, due to charge transfer interactions and resonance stabilization in linked aromatic rings versus single pyridinium ions. Although... 

The reaction begins with the protonation of one of the carbon-Ccirbon double bonds (see Figure 4-12) by the hydrogen ion from the HBr. A primciry or a secondary carbocation can be formed by this reaction. As seen in Organic Chemistry 1, a secondary Ccirbocation is more stable than a primary carbocation. Also, this secondary ccirbocation is even more stable because it s allylic and resonance stabilized. 

Monomer reactivities in anionic copolymerization are the opposite of those in cationic copolymerization. Reactivity is enhanced by electron-withdrawing substituents that decrease the electron density on the double bond and resonance stabilize the carbanion formed. Although the available data are rather limited [Bywater, 1976 Morton, 1983 Szwarc, 1968], reactivity is generally increased by substituents in the order... 

Two further mechanisms are known to trap electronic charge in thin films intermolecular and resonance stabilization. In resonance stabilization, electron attachment to a molecular center produces an anion in a vibrationally excited state that is then de-excited by energy exchange with neighboring molecules. When the initial anion ground state lies below the band edge or lowest conduction level of the dielectric, then the additional electron may become permanently trapped at the molecular site. In this case, a permanent anion is formed (e.g., the case of O2 [220]). Intermolecular stabilization refers... 

The generality of Scheme 1 is apparent by now 7r-distortivity and resonance stabilization are two coexisting properties of delocalized species which are constrained to relatively small distortions around a symmetric geometry. This situation that typifies conjugated jr-systems of carbon allows one to measure the two properties independently of each other. 

This process terminates the chain growth. It is an energetically favorable process because the resulting radical is allylic and resonance-stabilized ... 

Scheme 8 The role of formaldehyde in the formation of carbenium-iminium ion 17 and resonance stabilization of the latter...

The hydrogen on the nitrogen is removed by the base. This hydrogen is relatively acidic due to the electron-withdrawing effect of the bromine and resonance stabilization of the conjugate base by the carbonyl group. 

Q Draw resonance forms and propose mechanisms to explain the observed products and the enhanced rates of reactions involving conjugated reactants and resonance-stabilized intermediates. 

Application of the Wittig reaction in the carbohydrate field is accompanied by certain difficulties. A correct choice of the initial sugar components is the main problem, owing to the basicity of phosphoranes and, especially, to the drastically basic conditions employed with phosphonium ylides (2a). It is not surprising, therefore, that protected (acetalated and aeetylated) aldehydo sugars and resonance-stabilized phosphoranes were used at first,3-5 although partially protected, and even unprotected, aldoses were shown to be amenable to the reaction with various resonance-stabilized phosphoranes, thanks to the presence of the carbonyl form in the mobile equilibrium. The latter reactions, however, are extremely complicated (see Section IV, p. 284). 

There appears to be at least one factor, in addition to angular strain and resonance stabilization, which determines the relative importance of photoinduced decarbonylation, 6-cleavage, and ring expansion of cyclobutanones. It has already been demonstrated that increasing a-alkylation of cyclo-butanone results in an increase in the extent of cyclic acetal formation (i.e., ring expansion). Comparison of [64a] and [64b] reveals a similar trend. The 3-phenyl-cyclobutanones [141] and [144], however, provide the most spectacular effect of a-substitution (25). In both cases, a-cleavage occurs exclusively at the most highly sub-... 

Naphthyridines. Part II. Covalent Hydration, Electron-deficiency, and Resonance Stabilization in 1,6-Naphthyridines. 

In all the examples studied, the difference in the free energy between the anhydrous and hydrated species is 4 kcal/mole or less. Both electron deficiency and resonance stabilization are necessary for covalent hydration to be measurable. The necessity for electron deficiency is clearly shown in the following examples. The cation of... 

Cyclopentadiene itself is not aromatic because it is not fully conjugated. The cyclopentadienyl anion, however, is both aromatic and resonance stabilized, so it is a very stable base. As such, it makes cyclopentadiene more acidic than other hydrocarbons. In fact, the p/Ca of cyclopentadiene is 15, much lower (more acidic) than the p a of any C-H bond discussed thus far. 

Carbanions formed after proton abstraction from alkyl substituents a. to the ring nitrogen are inductively and resonance stabilized and undergo typical condensation reactions with a variety of electrophilic reagents. 

Food containing BI IT is preserved from oxidative degradation because O2 abstracts H-from the phenol group of BFIT instead of from the food. The resulting phenoxy radical is persistent and unreactive due to steric encumbrance and resonance stabilization. 

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